Improved method forming aluminum magnesium sheet



United States Patent 3,234,053 IMPROVED METHOD FGRMING ALUMENUM-MAGNESIUM SHEET Michael J. Pryor, Hamden, Conn., assignor to OlinMathicson Chemical Corporation, a corporation of Virginia No Drawing.Filed May 14, 1963, Scr. No. 280,443

Claims. (Cl. 148-115) This application is a continuation-in-part ofco-pending application Serial No. 162,986, filed December 28, 1961, byM. Pryor, now abandoned.

This invention relates to the formation of improved properties in sheetand strip aluminum-magnesium alloys by controlled cooling. Moreparticularly it relates to a process for the improvement of certainphysical properties of aluminum-magnesium alloys, which are suitable forbright anodizing in sulfuric acid after prior treatments such as bufiingand/ or bright dipping, by controlled cooling of selected portions ofthe sheets or strips, and is directed toward the achievement of greatlysimplifying the production of such material in a form highly suited tosubsequent bright anodizing processing.

Of the many sheet metal products formed of rolled metal, a largepercentage desirably have a surface appearance which is highly lustrousand generally pleasing to the eye, and also have functionalmetallurgical properties of hardness, formability, or other desirablecombinations of physical properties. The production of sheet aluminumhaving these esthetic characteristics combined with physical propertiesis a highly developed art and constitutes a very substantial portion ofthe existing capacity of the aluminum industry.

In the art of producing sheet and strip material from aluminumcontaining small additions of magnesium which are suitable for brightanodizing, serious difiiculties arise from the relationship of thecombined factors of hot rolling temperature and pressure. For example,suitable and desirable surface, as well as bulk, characteristics insheet and strip materials made from the aforementioned alloys isnormally achieved through the conjoint action of a moderate hot rollingtemperature combined with an essentially high separating force on thehot mill rolls. The factor of temperature is critical because of thenecessity of accomplishing the strain induced precipitation ofintermetallic compounds such as Mg Si and BMg Al at relatively lowtemperatures so that a very fine dispersed precipitate is obtainedrather than at higher temperatures such as 700 to 800 P. where anundesirable coarse dispersion of these compounds is obtained.

However in the lower temperature range 450 to 650 extreme pressure isrequired to make significant Warm reductions due to the substantialstrain hardening of these alloys. More specifically as hot rollingtemperature decreases, particularly with regard to aluminum-magnesiumalloys, separating force on the hot mills increases sharply requiringhot mills of great power and stiffness. In addition, extremely carefulcontrol of temperature at any given reduction, as well as high eflicientremoval of oxide coatings from the work roll surface and extremelyeffective lubrication are needed. As a result of these factorscommercial production of acceptable bright sheet products by lowertemperature hot rolling tends to be low in comparison to the rate ofproduction resulting from high temperature hot rolling unless costlymultiple stand tandem mills of great power, stiffness and cost areavailable. On the other hand high temperature hot rolling frequentlyproduces a product substantially inferior in bright anodizingcharacteristics to that produced by low temperature hot rolling and thisresults in a commercially unacceptable degree of waste.

3,234,953 Patented Feb. 8, 1966 It has been recently found that theproperties obtained by hot rolling in the lower temperature ranges aredesirable or even necessary only in a surface layer of the metal sheetor strip, this surface layer frequently constituting only a very minorportion of the total thickness of the sheet. The remainder of thethickness, which is not involved in the bright anodizing process, isaccordingly, susceptible to maintaining the physical properties whichare desirable for greatest ease of hot rolling and for lowest cost inthis operation. Accordingly, an article is achieved which possesses agradient of physical properties through its thickness which result frommore suitable processing than heretofore known and which yield desirablecharacteristics for all portions of the thickness of the sheet or strip.

In connection with this processing it is well known that there are twotypes of constituents or intermetallic compounds which influence thebright anodizing characteristics of aluminum-magnesium alloys. Firstly,there are those which are nearly insoluble during anodizing and becomeincluded in the anodic film; these obviously reduce the light which istransmitted through the anodic film, and include such constituents asFeAl MnAl and uAlFeSi. Maximum bright anodizing characteristics areobtained if these constituents are present in a coarse particulate form.

The other type of constituents which influence bright anodizingcharacteristics are those which are soluble during anodizing such as MgSi and ,8 phase Mg Al Their distribution affects the roughness of thereflecting oxide film-metal interface; but because they dissolve duringanodizing they do not interfere with the oxide film clarity to asignificant degree. A given quantity of these constituents dispersed infine particulate form produces a smooth oxide film-metal interfacehaving a high specular reflectivity. A similar quantity of theseconstituents dispersed in coarse particulate form produces a roughenedoxide metal interface having a low specular reflectivity and poor imageclarity.

It is also well known that the distribution of soluble constituents ofMg Si and Mg Al can be controlled in the hot rolling process. This isaccomplished in an intermediate temperature range of 450 to 700 F. underthe conjoint action of temperature and roll deformation. The lower thetemperature, the finer will be the dispersion of the constituentparticles and therefore the higher the reflectivity after brightanodizing.

In view of the above mentioned factors, and others that Will becomeapparent hereinafter, it is a principal object of this invention toprovide an improved method forming aluminum-magnesium sheet or stripmaterial which is suitable for bright anodizing.

It is another object of this invention to provide a method of formingaluminum-magnesium sheet or strip material having metallurgicalcharacteristics in at least a surface layer thereof which are suitablefor bright anodizing.

It is a still further object of this invention to form a brightanodizing aluminum-magnesium sheet or strip material having a gradientof metallurgical properties throughout the thickness thereof which aredesirable both in subsequent bright anodizing.

It is still another object of the present invention to provide a processof manufacturing bright anodizing aluminum-magnesium sheet or stripmaterial which facilitates the achievement of certain desiredmetallurgical properties by hot rolling in low temperature ranges withsubstantially reduced separating forces on the hot mill rolls.

Other objects and advantages of the present invention will becomeapparent from a consideration of the following description and examples.

In accordance with the principles of this invention it has beendiscovered that an improved method of manufacturing bright anodizingaluminum-magnesium sheet material is achieved, in which the sheet has ahigh thermal conductivity and has magnesium present in the form of atleast one precipitated intermetallic magnesium compound.

uniformly dispersed in fine particle size in a surface layer of thesheet, by (1) providing a sheet of aluminum-magnesiu-m alloy havingmagnesium in the form of at least one intermetallic magnesium compoundas a soluble constituent of the alloy, (2) dissolving the constituentinto solid solution in the alloy (3) establishing a thermal gradientthrough the thickness of said sheet,and (4) precipitating theconstituent through the conjoint action of said thermal gradient andmechanical deformation to produce, in a surface layer only of saidsheet, a fine particle size uniform dispersion of said constituent and acoarse particulate dispersion of said constituent in the remainder ofthe thickness of said sheet. As an adjunct of this process we havediscovered a novel article of manufacture in the form of an integralbright anodizing aluminum-magnesium sheetwhich is characterized byhaving a gradient of metallurgical properties throughout the. thicknessthereof, the gradient being constituted by at least one surface layer ofthe sheet having magnesium in the form of at least one precipitatedintermetallic magnesium compound uniformly dispersed throughout thislayer in fine. particle sizewith the remainder of the thickness of thesheet having this compound dispersed in .a coarse particle size inrelation to the aforementioned fine particle size.

Aluminum alloys suitable for subsequent bright anodizing after beingformed into sheet or strip material fall generally into two categories,those containing around 2% to 1.8% magnesium and those containing around2% to 3.2% magnesium, both with Fe and Si as impurities in amounts lessthan .4%. Included within the first group are alloys 5257 (.2%-.6%)magnesium, 5357' (.8%1.2% 5457 (.8%1.2% 5557 (.4%-.8% 5657 (.6%-1.80%),5757 (.6%1.80%), 5857 (.5%8%), 5957 (.4%.8% among the second group aresuch alloys as 5252 '(2.2%-2.8%), 5652 (2.2%-2.8%), and 5053 (3.2%).These aluminum-magnesium alloys contain magnesium in sufficient quantityto form .intermetallic magnesium compounds which are soluble in hightemperature ranges. In the case of the .2% to 1.8% mag: nesium group theoperative intermetallic compound is Mg Si which ,is'soluble in thetemperature range of 750 to 850 F. and is precipitated upon cooling tobelow this temperature range. In the case of the 2% to 3.2% magnesiumgroup two intermetallic compounds of magnesium are formed, Mg Si whichis soluble in the temperature;

range previously stated, and Mg Al which is soluble in the temperaturerange of 850 to 950 F. and which precipitates upon subsequent coolingbelow this latter temperature range. In either case. the lowertemperature precipitation of these compounds is accelerated bydeformation at lower temperatures.

After casting the ingot and scalping it, the alloy is reheated prior tobreakdown hot-rolling, to a temperature of sufficient magnitude toredissolve the intermetallic.

magnesium compound into solid solution in the base metal. Thitemperature is within the range of 750 to 950 F. depending upon which ofthe aforementioned groups of aluminum-magnesium alloys is selected forprocessing. Desirably the scalped process ingot is heated toapproximately 850 F. in order to drive the Mg Si into solid solution, orto approximately 900 E, which tem-, perature is required to drive the MgAl into solid solution, in the event that one of the 2% to 3.2%magnesium alloys is selected for the processing. Within these tem-.perature ranges the alloys have a high degree of plastic flow andaccordingly can be rolled with considerably high reductions per passwith far less separating force on the hot mills than would be requiredfor corresponding rolling rolling in the tandem mill the material isreduced to lower 4 gages and the alloy cool within the temperature rangewhere precipitation of magnesium intermetallic compounds can occur underthe conjoint action of temperature and roll pressure. As the alloybecomes cooler its resistance to deformation or reduction increasesstrongly thereby limiting the degree of reduction in thickness that canbe obtained in a single pass; This condition becomes more severe thelower the bulk temperature of the strip. Attempts to alleviate thisresistance to, deformation by final hot rolling at'a high-er bulktemperature, where resist ance to deformation isless, 'is accompanied inthese aluminum-magnesium alloys by a coarser dispersion of precipitatedintermetallic compounds and poorer bright= anodizing characteristics. a

To overcome this diificulty, and to obtain the'desired fine dispersionof magnesium intermetallic compounds which result in excellent brightanodizing characteristics, While at the same time maintaining theproduction advantages of higher temperature hot rolling, the sheet orstrip is subjected to a very. rapid one side cooling process immediatelybefore the final hot reduction or reductions.

It has been found that heat can be removed rapi dly from one side of acontinuously advancing high temperature strip to establish a steeptemperature gradient through the thickness of the strip immediatelybefore'its'entry into the last hot reduction. This gradient isestablished by the application of a cooling medium directed at anextremely high velocity to the side of the strip which it is desired to.

be'cooled, and must be maintained .until the strip enters the .roll biteand becomes plastically deformed. It shouldv be noted thatthealuminum-magnesium .alloys suitablef'for use in connection withthisinvention havean extremely high thermal conductivity, in the order ofB.t'.u. per

hour per square f ootper degree Fahrenheit; accordingly in order todevelop an effective thermal differential at the instant of reduction bythe rolls it is necessary to use a:

high capacity cooling systemvsuch as .that described in copendingapplication for patent S.N. 156,119, filed No-' vember 30,1961, assignedto a common assignee as the instant application. According to the novelcooling process of that application a liquid medium is expressed from anorifice as a highvelocity jet-which is directed against the surface tobe cooled substantially perpendicular thereto. Rapid cooling isachieved. even though vaporization of the cooling liquid form on the hotmetal surface,

I thereby constituting a vaporons thermal barrier, due to the fact thatthe cooling medium moves at such a high velocity that it penetrates :thevbarrier and brings the cooling liquid into direct contact with the hightemperature surface.

Since theabove mentioned alloys, which are used in connection with thisinvention,;have -arrelatively high thermal conductivity, specializedproblems arise with respect to maintaining the thermal differentialuntil the moving strip enters the roll bite. Withirespect to the rate ofcooling of the surface-layer, it hasbeen found that in a metal sheet orstrip in the order of-a quarter inch or less, itiis possible to developa temperature gradient of as much as 400 F. where the temperature of thesheet was originally at 800 F. by using a spray which removes heat at acoefii'cientatabout 8,000. 3.t.u. per hour per square foot ,per degreeFahrenheit. In order to establish such a heat transfer coefiicient atthe metal surface, spray must be delivered at a volumetric .rate ofPPIOXimately 10 gallons per minute under a pressure of at least 300 1psi. through a spray developing nozzle located at about 12" from the hotmetal surface Lower. temperature coeflicients cause lower-gradients tobe established. Thus where the coefficient is about4,000 in the aboveexample a temperature gradient of about 225 F. is developedin a sheetinitially at 800 F.

The development of temperature differentials in the range of 200 to 400F. is necessary in order to, establish the gradient of metallurgicalproperties ultimately desired inthe .finished sheet suitable for brightanodizing. Thus. the effect of the :one side cooling and maintainingthis temperature differential until the strip is subjected todeformation is to promote rapid precipitation of the intermetalliccompound, or compounds as the case may be, as a fine particle sizeuniform dispersion in a substantially cooler thin surface layer only ofthe sheet when the sheet is subjected the mechanical deformation, whilesimultaneously permitting the precipitation to occur, upon deformation,as a coarse particulate dispersion in the much hotter bulk of thethickness of the sheet. In the case of the .2% to 1.8% magnesium alloys,for example, precipitation of fine Mg Si will occur within thetemperature range of 450 to 550 I? and under the exemplary conditionsset forth above this will produce a surface zone of approximately /2 to1 mil in thickness, in a final sheet of L050" in thickness, this .0005to .001 thickness being all that is required in the final sheet togenerate good brightness after bright dipping (electrobrighting andpolishing) and anodizing, with or without mechanical bufiing. Normallyabout of the thickness of the final sheet is required as the surfacelayer for these operations. Con-fining the desirable fine precipitatesof magnesium intermetalliccompounds to the outer 10% of the strip duringhot rolling results in an average bulk temperature that is close to'thatof the strip before the application of the coolant. Little averagetemperature drop results under .these conditions which results in easyhot rolling with large permissible reductions which is characteristic ofthe unquenched strip with the coarse dispersion of magnesiumintermetallic compounds and the poor bright anodizing character istics.

'In the case of the 2% to 3.2% magnesium group of alloys which containnot only Mg Si which is precipitated in the above-mentioned temperaturerange, but also 'Mg Al which is fully precipitated in a somewhat highertemperature range in the order of 550 to 700 F. A similar depth ofprecipitation is desired with these latter alloys as with the former.

'It is usually not advisable to develop two side temperature gradientson metal substantially less than a half inch in thickness Where themetal has a high temperature coefficient because substantial bulkcooling then occurs and resistance to roll deformation increasessharply. Distinct advantages are readily obtained, however, from twoside cooling of thicker metal .sheets during rolling operations becausethe roll acts-on the lower temperature surface by the reduction producedwhile a given mill roll separating orceis that characteristic of thehigh temperature core.

Regardless of whether one or both sides of the strip are cmled, thestrip rolls as a strip at a higher temperature but he finished producton the surface layer, which is substantially cooler than the bulk of thestrip, crystallizes during hot rolling inamanner characteristic of thelower temperature range to produce the desired fine particle sizeuniform dispersion of precipitate. It is noteworthy that usually,although not necessarily, the surface subjected to the high pressurecoolant will be the upper face of the strip since this is less subjecttomechanical scratching and other handling damage.

.The high velocity coolant, under transient conditions only leads to asteep temperature gradient in approximate'ly the surface 10% of'thestrip as mentioned before, with little bulk cooling of the remainder ofthe metal. High hot rolling speeds in the order of 250 feet per minutefavor the maintenance of these transient conditions until the stripenters the roll bite. Since the separating force on the hot mill is afunction of the average temperature of the strip at the instant of stripdeformation this temperature differential will enable similar hotrolling to be accomplished at speeds and reductions characteristic ofthe average higher temperature of the strip. However, the upper surfacelayers being as much as 200 to 400 F. cooler than the bulk" materialwill recrystallize during hot rolling in a manner characteristic of thelower temperature range, thereby producing the uniform fine particlesize dispersion of the intermetallic magnesium compounds.

In further connection with the establishment of the thermal gradientthroughout the thickness of the sheet,

it .should be pointed out that, while it is preferred to employ a veryrapid cooling system such as the high pressure spray system mentionedabove, it will be apparent that other systems may be employed. Forexample, the desired thermal gradient may be established by applying acooling effect to one surface of the sheet while simultaneously applyinga comparably efficient heating effect to the opposite surface.

Precipitation of the intermetallic magnesium compounds in the desiredparticle size and uniformity of dispersion is effected through theconjoint action of temperature differential and mechanical deformationof the hot sheet or strip. Customar'ily, although not necessarily, thisdeformation is accomplished by pressure rolling of the sheet immediatelyafter the one side cooling to effect a reduction in the thickness of themetal. Where the control of the magnesium compound precipitation is ofprimary concern, the pressure rolling reduction should be accomplishedon the final hot rolling pass. This is generally, although notnecessarily, true where other considerations such 'as grain size andtexture control are of primary importance, these factors being discussedin more detail hereinafter; however, of these two stages of hot rolling,th ultimate pass is the most important and the most critical in order toachieve the desired metallurgical prop- In order to obtain the desiredparticle size and uniformity of dispersion of the magnesium precipitatesin the finished pro-duct, it is desirable that the final hot rollingreduction occur while the outer surface layer of the sheet or strip isat a temperature within the range of 450 to 550 F. in the case of the.2% to 1.8% magnesium alloys, and within the range of 550 to 700 F. inthe case of the 2% to 3.2% magnesium alloys. By

effecting the hot rolling reduction with the outer surface layer at atemperature within these ranges with the bulk of the material stillwithin the higher temperature range of 750 to 950 -F., a differential orgradient of metallurgical properties is established within the thicknessof the material which yields desirable characteristics of the finishedproduct for the purpose of both the surface layer appearance upon brightanodizing and greatest ease and economy "of hot rolling. Morespecifically under these circumstances of temperature differentialduring final hot rolling, the intermetallic magnesium compound orcompounds are precipitated in a manner characteristic of low temperaturerolling to yield the desirable very fine particle size uniformdispersion, while retaining its coarse particle size duringprecipitation which is characteristic of higher temperature rolling. Atthe same time the separating force on the rolls need only be that whichis character istic of that required for rolling the strip with theentire thickness thereof at the higher temperature level.

For purposes of definition, distinction between fine and coarse particlesize is made by defining fine particle size as that which is notresolvable under a conventional optical microscope at a magnification of500 diameters.

In order to effect the final hot rolling of the strip with the desiredthermal gradient established therein, it is necessary that a minimumamount of time elapse between the development of the temperaturegradient and the subsequent rolling. It has been found that a suitablysteep temperature gradient in the order of 150 to 250 F., but not lessthan F., nor more than 350 1 should be established not lower than .4second and preferably .1 second before the strip enters the roll bite.It is necessary to confine this time lapse to the shortest possibleperiod due to the high thermal conductivity of the metal sheet or strip.If longer periods of time elapse the temperature gradient will becomevery shallow; this may in some cases be offset by the one side heatingand one side cooling as mentioned above. In either case, however, theaverage temperature of the strip should not 7. be reduced more than 50F. from a desirable and economical average hot rollingtemperature.

As a practical matter, in order to effectthe final hot rolling reductionin as short a time as possible after the.

establishment of. the desired temperature gradient, the cooling isapplied at a point close as possible to the contact point of the stripwith the hot mill, and this is generally in the order of 1 foot or less.Of course, it should be kept in mind that this distance is a function ofthe linear speed of the strip as it passes through the hot mill, andsince the temperature differential is transient, i.e., can be maintainedfor only very brief periods of time, it isdesirable to maintain a fairlyhigh m-ill speed on the strip. At these short distances from thestriphot roll contact point, water soluble oil or other hot mill coolinglubricant used as a cooling medium is suitable for the achievement ofthe temperature gradient. HOW! ever, the use of other cooling fluidssuch as Water is contemplated where additional measures are employed toremove the cooling liquid, as by air or water Wipe devices, before thestrip enters the hot roll surface.

The following are illustrative examples of this invention:

Example I A 5457 aluminum-magnesium alloy was direct chill cast into a20" thick ingot, cooled to room temperature and scalped to removesurface imperfections. It was then reheated to 875 F., and reduced in areversing mill.

from approximately 19% inches to approximately half inch strip. Thestripwas then fed into the-first stand 01' a tandem continuous hot millpress for a further re duction to a gage suitable for processingunderthe princi- Since the solution heating was unnecessary. Between thepenultimate and ultimate hot rolling passes, the strip was subjected toa high velocity stream of a cooling medium on the upper surface thereofto cool-the surface and the adjacent of the thickness of the metal toabout 500 F., thereby establishing the desired thermal gradient. Thecooling medium nozzles were located aboutone foot away from the surfaceof the strip and directed. at right angles thereto. The distance betweenthe point of application of the cooling medium and the hot roll nip wasabout 10 inches.

At a linear speed of about250'ft. per minute, the strip passed into thefinal stand of hot rolls and subjected to a further reduction to about.20 to precipitate the, intermeta-llic compound under the conjointaction of temperatur-e differential and mechanical deformation due toroll pressure. The sheet was then examined and found to have the desiredgradient of metallurgical properties through its thickness of fineparticle size uniform precipitate dispersion and excellent brightanodizing characteristics in the surface 10%, with a relatively coarserparticulate precipitate dispersion and poorer bright anodizingcharacteristics in the unquenched remainder of the thickness.

Example II A sample of 5252 aluminum alloy was prepared and i processedin the manner of Example I with the exception that the bulk temperatureof the metal was at 875 F. at the moment of the final hot rolling andthe. surface layer temperature after one side cooling was at 600 F. Goodresults were obtained.

The foregoing method of the present invention is particularly adapted tothe manufacture of sheet "or strip' or zone adjacent at least one sideof' the, shee t or strip and the .bulk remainder: of the .materiaL. The.surface' layer is characterized .by having .a .fine.v particle. sizeuniform dispersion of the .intermetallic'magnesium compound, 'orcompounds'as the case maybe, precipitatedv invention have application infields other thanthe produce tion of sheet; or. strip materialfsuitablefor'subsequent bright anodizing. .For examplejan'inherem characteristicof the one sidecoolir'ig process of this inventionis that the surfacelayer. of the' material, being as much 'as 350? cooler than. the bulkmaterial during final hot rolling.

recrystallizeslduring hot'rol-ling in a manner characteristic of thelower temperature,"therebyproducing a fine grain size and so called.warrn rolled. texture'in'the-base metal in addition/totheprecipitate.characferistics desired for bright anodizing. Thesecharacteristics of fine grainsize' and warm rolled texture, while theypromote'maxirnuin bright anodizing qualities, are beneficial to theperformance of aluminum alloys subsequently used in the produc-, tion ofspecificarticles under the process described in"U.S.

Patent No. 2,690,002.I I

This inventionmay be embodied in other forms. or

carried out in 'other ways without departing from the: spirit oressential characteristics thereof. The present embodiment of theinvention 'is therefore to be considered i.

in all respects as illustrative and not restrictive, thev scope of theinvention being indicaed by the appended claims and all changes whichcome within the meaning and range are intended to be embraced 1" ofequivalency of the claims therein.

What I claim and desire to secure .by Letters'Patent is: 1. A processvofproducing an aluminum-magnesium sheet having a high thermalconductivity and having magnesium'presentin the form of "at least oneprecipitated intermetallic magnesium compound uniformly dispersed infine particle size in a surface layer of said sheet, said processcomprising the steps of A. providing a sheet of aluminum-magnesium alloyhaving magnesiumin the form'of at least one intermetallic .magnesium'compound as a soluble con stituent of said alloy, 7

alloy by heating said sheet, C: establishing a thermal gradient throughthe thickness ofsaid sheet characterized 'by developing in asur- B.'dissolving said constituent into solid solution in said:

face layer only of said. sheetp a temperature lower than that developedin the remainder of the thickness of said sheet, andv D. precipitatingsaid constituent through the conjoint action-.of-said thermal gradientand mechanical deformation to produce,.in said surface layer only, afine particle size uniform dispersion of said constituent-and acoarseparticulate dispersion of said constituentin said remainder of thethickness of sai sheet. 2. A .process of producing an aluminum-magnesiumsheet having a high thermal conductivity and having magnesium .presentin the form of at least one precipitated intermetallic magnesiumcompound. uniformly dispersed in fine particle size in a surface layerof said sheet, said.

process comprising the steps of A. providing 'a sheet ofaluminum-magnesium alloy having magnesium in the form of at least oneintermetallic magnesium compound .as. a soluble constituent of saidalloy,

B. dissolving said constituent into solid solution in said alloy byheating said sheet substantially uniformly through the thicknessthereof,

C. establishing a thermal gradient through the thickness of said sheetby rapidly cooling at least one surface of said sheet whereby saidthermal gradient is characterized by a surface layer only of said sheetbeing at a substantially lower temperature than the remainder of thethickness of said sheet, and

D. precipitating said constituent through the conjoint action of saidthermal gradient and mechanical deformation by pressure rolling toproduce, in said surface layer only, a fine particle size uniformdispersion of said constituent and a coarse particulate dispersion ofsaid constituent in said remainder of the thickness of said sheet.

3. A process of producing an aluminum-magnesium sheet having a highthermal conductivity and having magnesium present in the form of atleast one precipitated intermetallic magnesium compound uniformlydispersed in fine particle size in a surface layer of said sheet, saidprocess comprising the steps of A. providing a sheet ofaluminum-magnesium alloy having magnesium in the form of at least oneintermetallic magnesium compound as a soluble constituent of said alloy,

B. heating said sheet substantially uniformly through the thicknessthereof thereby dissolving said constituent into solid solution in saidalloy,

C. rapidly cooling at least one surface of said sheet therebyestablishing a thermal gradient through the thickness of said sheetcharacterized by a surface layer only of said sheet being at asubstantially lower temperature than the remainder of the thickness ofsaid sheet, and

D. pressure rolling said sheet immediately after said surface cooling toprecipitate said constituent through the conjoint action of said thermalgradient and mechanical deformation as a fine particle size uniformdispersion in said surface layer only and in a coarse particulatedispersion in said remainder of the thickness of said sheet.

4. A process of producing an aluminum-magnesium sheet having a highthermal conductivity and having magnesium present in the form of atleast one precipitated intermetallic magnesium compound uniformlydispersed in fine particle size in a surface layer of said sheet, saidprocess comprising the steps of A. providing a sheet ofaluminum-magnesium alloy having magnesium in the form of at least oneintermetallic magnesium compound as a soluble constituent of said alloy,

B. heating said sheet substantially uniformly through the thicknessthereof to a temperature within the range of 750 to 950 F. therebydissolving said constituent into solid solution in said alloy,

C. rapidly cooling at least one surface of said sheet to a temperaturewithin the range of 450 F. to 700 F. thereby establishing a thermalgradient through the thickness of said sheet characterized by a surfacelayer only of said sheet being at a substantially lower temperature thanthe remainder of the thickness of said sheet, and

D. pressure rolling said sheet immediately after said surface coolingand while said thermal gradient is at least F. to precipitate saidconstituent through the conjoint action of said thermal gradient andmechanical deformation as a fine particle size uniform dispersion insaid surface layer only and in a coarse particulate dispersion in saidremainder of the thickness of said sheet.

5. A process as set forth in claim 4 wherein A. said alloy containsmagnesium in the form of Mg -Si as a soluble constituent thereof,

B. said heating is within the temperature range of 750 to 850 F., and

C. said surface cooling is within the temperature range of 450 to 550 F.

6. A process as set forth in claim 4 wherein A. said alloy containsmagnesium in the form of Mg Si and Mg Al as soluble constituentsthereof,

B. said heating is within the temperature range of 850 to 950 F., C.said surface cooling is within the temperature range of 550 to 700 F.

7. As an article of manufacture, an integral aluminummagnesium sheetcontaining from 0.2 to 3.2% magnesium, balance essentially aluminum,said sheet being characterized by having a gradient of metallurgicalproperties through the thickness thereof, said gradient comprising atleast one surface layer of said sheet having magnesium in the form of atleast one precipitated intermetallic magnesium compound uniformlydispersed throughout said layer in fine particle size with the remainderof the thickness of said sheet having said compound dispersed in coarseparticle size relative to said fine particle size.

8. An article as set forth in claim 7 wherein said precipitatedmagnesium compound is Mg Si.

9. An article as set forth in claim 7 wherein said precipitatedmagnesium compounds are Mg Si and Mg Al 10. An article as set forth inclaim 7 wherein said surface layer comprises approximately 10% of thethickness of said sheet.

References Cited by the Examiner UNITED STATES PATENTS 2,177,711 10/1939Graham 148-32 3,042,555 7/1962 George et a1 148-325 3,164,494 1/1965English 148-315 DAVID L. RECK, Primary Examiner.

H. F. SAITO, Assistant Examiner.

1. A PROCESS OF PRODUCING AN ALUMINUM-MAGNESIUM SHEET HAVING A HIGHTHERMAL CONDUCTIVITY AND HAVING MAGNESIUM PRESENT IN THE FORM OF ATLEAST ONE PRECIPITATED INTERMETALLIC MAGNESIUM COMPOUND UNFORMLYDISPERSED IN FINE PARTICLE SIZE IN A SURFACE LAYER OF SAID SHEET, SAIDPROCESS COMPRISING THE STEPS OF A. PROVIDING A SHEET OFALUMINUM-MAGNESIUM ALLOY HAVING MAGNESIUM IN THE FORM OF AT LEAST ONEINTERMETALLIC MAGNESIUM COMPOUND AS A SOLUBLE CONSTITUENT OF SAID ALLOY,B. DISSOLVING SAID CONSTITUENT INTO SOLD SOLUTION IN SAID ALLOY BYHEATING SAID SHEET, C. ESTABLISHING A THERMAL GRADIENT THROUGH THETHICKNESS OF SAID SHEET CHARACTERIZED BY DEVELOPING IN SURFACE LAYERONLY OF SAID SHEET, A TEMPERATURE LOWER THAN THAT DEVELOPED IN THEREMAINDER OF THE THICKNESS OF SAID SHEET, AND D. PRECIPITATING SAIDCONSTITUENT THROUGHT THE CONJOINT ACTION OF SAID THERMAL GRADIENT ANDMECHANICAL DEFORMATION TO PRODUCE, IN SAID SURFACE LAY ONLY, A FINEPARTICLE SIZE UNIFORM DISPERSION OF SAD CONSTITUENT AND A COARSEPARTICULATE DISPERSION OF SAID CONSTITUENT IN SAID REMAINDER OF THETHICKNESS OF SAID SHEET.